Genes regulating reproductive success in laboratory mice have provided important insights into the molecular and developmental processes underlying mammalian reproduction and have served as models for studies of human infertility. Traditional strains of laboratory mice, however, do not show the degree of variation in reproductive traits that one sees in nature. In contrast, a close relative to the laboratory mouse, the genus Peromyscus, exhibit striking differences in testes size, ejaculate traits, sperm morphology and swimming performance between species. This variation is likely due to the extreme divergence in mating system within the genus. In species where females mate multiple times over a breeding season, there is intense competition between ejaculates of different males for fertilization of her eggs. Accordingly, there is strong selective pressure on male reproductive traits that improve fertilization success under competition in promiscuous species. The proposed study is designed to exploit this natural variation in male reproductive traits to reveal genes that contribute to fertilization success, and capitalizes on a wealth of genomics tools available in mice. The two sister species on which this study will focus, P. maniculatus (promiscuous) and P. polionotus (monogamous), are interfertile, allowing one to measure the intensity of sperm competition both within and between species. Using laboratory pairing experiments in which either males or females are paired with multiple partners, I will measure variation in male reproductive success to test hypotheses on the adaptive significance of the observed phenotypic differences. I will then use a genetic mapping approach that combines anonymous markers and candidate genes to identify genetic regions and ultimately genes that contribute to male fertilization success. This experimental design allows us to directly link phenotypic differences in sperm traits with genotypic variation. The broad goal of this research is to use this unique system to uncover the genetic basis of male reproductive traits influenced by sperm competition, thereby expanding our understanding of reproductive biology and male infertility. Relevance: This research is of direct relevance to the mission of NIH because it will identify and molecularly characterize traits affecting male reproductive success. This approach will likely uncover genes different from those regulating fertility in laboratory mice, which do not show the extreme divergence in mating system that Peromyscus does, or in humans, where such controlled experiments are not possible. The identification of such genes may provide important insights into male infertility, a problem encountered by over 10% of couples.